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It’s a slasher film scenario playing out in nature. Harbour seals use their whiskers to follow underwater vibrations rippling away from gills of fish so they can home in on prey.

Until now we didn’t know how seals manage to locate and catch bottom-dwelling fish that are hidden beneath the sand.

“We have solved a longstanding riddle,” says Wolf Hanke, the biologist at the University of Rostock in Germany who led the study. “These flatfish are very cryptic. They burrow in the ground and they’re covered with sand or silt, but the seals still grab them.”

The only way to avoid being eaten would be to stop breathing until the seal swims on – although it’s not clear if flatfish do this.

Wonderful whiskers

Flatfish constitute up to 70 per cent of the diet of harbour seals, so being able to detect the fish is key to the seals’ success. After watching a hunting seal, with a camera attached to it, pinpointing flatfish with no visual giveaways to their position, Hanke and his colleagues set out to figure out how the seals do it.

To snap up the fish, seals use their whiskers to follow trails in the water.

“To see a seal following an underwater wake is amazing,” says Colleen Reichmuth, a marine biologist at the University of California, Santa Cruz. “It’s as if the animal is pulled by an invisible thread.”

The skill . Reichmuth was part of a team that developed a whisker array to mimic a seal’s ability to pinpoint underwater disturbances.

“It has a variety of purposes, but the most obvious one is related to military defence, being able to detect and track moving objects underwater and being able to do it potentially quietly without using active sonar,” she says.

Blindfolded seals

We already knew that seals follow the ripples that swimming fish create in the water, but it was unclear whether seals could detect the slight motion from a stationary fish’s breathing over the “noise” of ocean swells and the water movement caused by the seal’s own swimming.

Hanke and his team monitored three male seals – Luca, Henry and Filou – in a netted enclosure in the Baltic Sea that had been outfitted with eight nozzles that release water��at an angle and rate that mimics the breathing patterns of flatfish.

The seals were blindfolded for half of the trials. When they were able to see, all the seals found the nozzle emitting water currents with at least a 90 per cent success rate. When blindfolded, they were all still able to identify the correct nozzle with at least 75 per cent success.

The researchers also covered the seals’ faces and whiskers with a mask. In those trials, the animals didn’t find any of the active water��nozzles when searching, and mostly drifted on the water’s surface.

Don’t take a breath

It’s like an underwater slasher film, in which the flatfish’s best bet is to hide out and hold its breath until the killer has gone. Hanke says it is possible for flatfish to hold their breath, but it’s not clear if they do it when seals approach.

“When partially immersed in sand on the ocean bottom, many flatfish leave their eyes sticking out, and with 360 degrees of vision they would very likely see a seal swimming towards them,” says Alex Schreiber, who studies flatfish at St Lawrence University in Canton, New York.

“However, unless they have evolved a ‘stop respiring in the presence of a predator’ response,” he says, “they may assume they are safely camouflaged and be unaware of the seal’s ability to detect their respiration.”

Schreiber says we can’t assume either way. “If the flatfish have not evolved this response, then the seals are successfully exploiting a flatfish weakness, and have an advantage.”

Read more: First video footage of seal drowning and eating a pup

Correction��[1/19, 8 pm]:��A previous version of this story said that the nozzles emitted air, not water. This has been corrected.